A “mechanical wave” is a disturbance that propagates through a medium due to the restoring forces it produces upon deformation of the medium. Solids, liquids, gases, and gels are examples of media through which a mechanical wave may travel.
If desired, the energy of a mechanical wave can be exploited to deform and potentially fracture an object placed in the medium. For example, high intensity compression pulses (i.e., a brief wave of great amplitude) can be sent in the body of a patient to break a kidney stone apart.
One protocol for kidney stone destruction consists of emitting a compression pulse having a sufficient amount of energy for traveling through the body, reaching the stone, and potentially rupturing the kidney stone upon contact. Machines used in medical kidney stone destruction are known in the art as lithotripters. The external lithotripters send externally-applied, focused, high-intensity compression pulses toward the kidney stone. As the high intensity compression pulses travel through the body of the patient, non-linear effects eventually deform these pulses into shockwaves. When a shockwave encounters a non-homogeneity such as the kidney stone, a relatively large amount of energy is transferred from the shockwave to the kidney stone in a (relatively) very short period of time. Ideally, this energy transfer is sufficient to break enough of the bonds between the stone particles to destroy the stone. With external lithotripters, the location of the kidney stone within the body of the patient must be known in order to direct the high-intensity compression pulses toward the kidney stone.
Despite their widespread use, conventional lithotripters are cumbersome apparatuses. First, they have the drawbacks of potentially damaging tissue adjacent to the kidney stone and producing large kidney stone fragments. Second, they have a limited focal length. Occasionally, conventional machines even fail to fragment the hardest kidney stones. Finally, conventional lithotripters often require the inclusion of apparatuses such as fluoroscopy (x-ray) or ultrasound machines for locating the kidney stone.
Montaldo et al. ‘Generation of very high pressure pulses with 1-bit time reversal in a solid waveguide’, J. Acoust. Soc. Am. 110(6), December 2001 have developed a way to focus high amplitude pressure pluses at predetermined locations in a fluid. The system of Montaldo et al. works according to the time-reversal mirror concept, which exploits the temporal reversibility (or reciprocity) of the wave equation of motion. Reciprocity says that if the wave equation has a solution, the time reversal (using a negative time) of that solution is also a solution of the wave equation.
The system S proposed by Montaldo et al., shown in FIG. 1, is composed of seven small independent bi-directional piezoelectric transducers T glued to one end of an aluminum bar (waveguide), which acts as a reverberative cavity RC. The transducers T can both emit and receive mechanical waves. The walls of the reverberative cavity RC are in contact with the air while the end of the reverberative cavity RC distal to the transducers T lies in water. In their experiment, Montaldo et al. use a source placed in the water to emit a pressure pulse toward the reverberative cavity RC. The pressure pulse is, after propagation through the reverberative cavity RC, recorded by each of the transducers T. As it travels through the reverberative cavity RC, the pressure pulse P undergoes some deformation due to reverberations R inside the reverberative cavity RC, as described below. The transducers T convert the recorded pressure pulse into an electric signal. The signal of each transducer T is then time reversed and processed to excite the same transducer T. The mechanical waves produced by each transducer T propagate through the reverberative cavity RC, by reverberations R, toward the other end of the reverberative cavity RC, and emerge at that end thereof to produce a focused pressure pulse W2 (shown in FIG. 2) at the location of the source.
As shown in FIG. 2, when a mechanical wave W1 created by one or more of the transducers T is propagated inside the cavity RC, reverberations R at the wall of the cavity RC redirected it to the core of the cavity RC. The reverberations R are a consequence of the difference of acoustic impedance between the reverberative cavity RC and the surrounding air. Since the wall reverberations R are with almost no energy loss, the mechanical wave W1 can travel inside the reverberative cavity RC without undergoing major attenuation. Each reverberation R creates the illusion of having originated from a virtual transducer VT. The assembly of these virtual transducers VT is perceived by an observer at a focal point FP as a source of great dimension, although only a limited number of real transducers RT is used.
As a consequence, the technology proposed by Montaldo et al. uses a limited number of low-power transducers to temporally and spatially concentrate trains of low amplitude waves in order to obtain a high amplitude and short-lasting focused wave. The spatial focalisation is made possible by the reverberating nature of the cavity while the temporal compression is made possible by the time reversal operation. Montaldo et al. sends the pulses at predetermined locations which correspond to locations where a source was originally positioned.
Montaldo et al.'s device reaches some limits, especially when applied to lithotripsy. A simple calculation can show that their proposed device is not capable of reaching focal distances compatible with applications where the target is typically remote from the wave emitting device. Further, to reach typical focal distances required for kidney stones destruction in human subjects, one would need to construct a device having an unrealistic number of transducers or else have the reverberative cavity of a cumbersome length or diameter. A device of such a size is far from Montaldo et al.'s main object which was to present a simple and compact alternative to current commercial lithotripters, and this probably explains why there is no evidence of the construction of such a device in literature.
Thus, in summary, in terms of the use of wave generators of high intensity acoustic pulses with possible applications in lithotripsy, it is believed that conventional technology has reached its limits in what it will allow, and the disadvantages noted above remain. While the wave generator proposed by Montaldo et al. may assist in ameliorating the situation, room for improvement would nonetheless still exist.